Although myotonic dystrophy (DM) is the most prevalent muscular dystrophy in adults, a more severe form of this neuromuscular disease is congenital DM (COM). While the two characterized types of adult-onset DM, DM1 and DM2, are caused by unstable microsatellite (CTG)n and (CCTG)n expansions in the DMPK and ZNF9 genes, respectively, CDM is only associated with >1,000 CTG repeats in the DMPK gene. The longterm goals of the proposed research are to elucidate the molecular events which result in CDM and use this information to develop novel therapeutic strategies for this disease. According to the RNA dominance model, DM is an RNA gain-of-function disease in which mutant DMPK and ZNF9 RNAs are retained in the nucleus where they inhibit the functions of the muscleblind-like (MBNL) proteins. MBNL proteins are alternative splicing factors and current evidence suggests that DM disease is caused by the retention of neonatal protein isoforms in the adult. The objective of this proposal is to use both knockout and transgenic mouse models to extend the RNA dominance model to CDM using four experimental aims. First, the hypothesis that specific Mbnl proteins are sequestered during embryogenesis by (CUG)n expansions will be tested using a new transgenic mouse model for CDM. Second, CDM is characterized by neonatal hypotonia and mental retardation so potential roles for Mbnl proteins in the regulation of pre-mRNA processing during embryonic muscle and brain development will be investigated. Third, CDM is only associated with DMPK CTG repeat expansions and previous studies have reported changes in the expression of DMPK, as well as the linked genes DMWD and SIX5, in CDM. Therefore, several mouse knockout lines will be generated to test the hypothesis that CDM results from coordinate loss of MBNL and one, or several, of these DMPK associated genes. Fourth, overexpression of DMPK RNA inhibits myogenic differentiation so the hypothesis that CDM results from co-sequestration of MBNL and DMPK RNA-binding proteins will be evaluated.